An ObservableValue is an entity that wraps a value and allows to
observe the value for changes. In general this interface should not be
implemented directly but one of its sub-interfaces (
ObservableBooleanValue etc.).

An implementation of ObservableValue may support lazy evaluation,
which means that the value is not immediately recomputed after changes, but
lazily the next time the value is requested. All bindings and properties in
this library support lazy evaluation.

An ObservableValue generates two types of events: change events and
invalidation events. A change event indicates that the value has changed. An
invalidation event is generated, if the current value is not valid anymore.
This distinction becomes important, if the ObservableValue supports
lazy evaluation, because for a lazily evaluated value one does not know if an
invalid value really has changed until it is recomputed. For this reason,
generating change events requires eager evaluation while invalidation events
can be generated for eager and lazy implementations.

Implementations of this class should strive to generate as few events as
possible to avoid wasting too much time in event handlers. Implementations in
this library mark themselves as invalid when the first invalidation event
occurs. They do not generate anymore invalidation events until their value is
recomputed and valid again.

Method Detail

addListener

Adds a ChangeListener which will be notified whenever the value
of the ObservableValue changes. If the same listener is added
more than once, then it will be notified more than once. That is, no
check is made to ensure uniqueness.

Note that the same actual ChangeListener instance may be safely
registered for different ObservableValues.

The ObservableValue stores a strong reference to the listener
which will prevent the listener from being garbage collected and may
result in a memory leak. It is recommended to either unregister a
listener by calling removeListener after use or to use an instance of
WeakChangeListener avoid this situation.

removeListener

Removes the given listener from the list of listeners, that are notified
whenever the value of the ObservableValue changes.

If the given listener has not been previously registered (i.e. it was
never added) then this method call is a no-op. If it had been previously
added then it will be removed. If it had been added more than once, then
only the first occurrence will be removed.